Abstract
Reversible inactivation of the cortex by surface cooling is a powerful method for studying the function of a particular area. Implanted cooling cryoloops have been used to study the role of individual cortical areas in auditory processing of awake-behaving cats. Cryoloops have also been used in rodents for reversible inactivation of the cortex, but recently there has been a concern that the cryoloop may also cool non-cortical structures either directly or via the perfusion of blood, cooled as it passed close to the cooling loop. In this study we have confirmed that the loop can inactivate most of the auditory cortex without causing a significant reduction in temperature of the auditory thalamus or other subcortical structures. We placed a cryoloop on the surface of the guinea pig cortex, cooled it to 2°C and measured thermal gradients across the neocortical surface. We found that the temperature dropped to 20–24°C among cells within a radius of about 2.5 mm away from the loop. This temperature drop was sufficient to reduce activity of most cortical cells and led to the inactivation of almost the entire auditory region. When the temperature of thalamus, midbrain, and middle ear were measured directly during cortical cooling, there was a small drop in temperature (about 4°C) but this was not sufficient to directly reduce neural activity. In an effort to visualize the extent of neural inactivation we measured the uptake of thallium ions following an intravenous injection. This confirmed that there was a large reduction of activity across much of the ipsilateral cortex and only a small reduction in subcortical structures.
Highlights
A significant challenge in auditory neuroscience lies in u nderstanding the descending projections from auditory cortex to subcortical auditory nuclei, and how these projections shape the processing of auditory information
We found that the area of reduced thallium uptake extended beyond the area of cortex marked by the 30° contour
The cooling device used in studies by Huang et al (2007) and Bardy et al (2009) differed from the cryoloop used in these experiments, presenting an additional confound to drawing comparisons between datasets
Summary
A significant challenge in auditory neuroscience lies in u nderstanding the descending projections from auditory cortex to subcortical auditory nuclei, and how these projections shape the processing of auditory information. Cortical inactivation is a useful approach for determining the extent of modulation by descending projections. There are a number of ways in which to inactivate cortex that differ in reversibility, magnitude, and duration of sustained effects. The cryoloop cooling technique for cortical inactivation produces a reversible, highly localized inhibition of neuronal activity when the temperature of neurones is reduced to
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